Does Exercise Improve Immunity?

'Runner's high' or an overall boost in mood and health are common in people who work out regularly, but is there scientific proof that exercise wards off illness?

Dr. Sapna Parikh found out that exercise definitely works, but only if it's in moderation.

She spoke with Dr. Brian Halpern, a sport medicine expert at Hospital for Special Surgery in Manhattan. He says moderate exercise to ward off disease should include twenty or so minutes of cardio that gets your heart rate up. "It kickstarts your immune system. It gets the cells working. It sends them after bacteria and viruses."

Exercising too much and for too long has the opposite effect: it can be more harmful than good. Research in mice found that high intensity prolonged workouts made them more susceptible to the influenza virus. For those already ill, extreme workouts made the illness worse.

"Too much exercise also stimulates hormones like cortisol and adrenaline that seems to impair the system," said Dr. Halpern.